A major unsolved question is how the main olfactory system detects and discriminates between complex, biologically relevant odors. Experiments performed during the present period of funding show that urine volatiles from mice differing at the class I major histocompatibility complex ('odortypes'), previously shown by others to be discriminated behaviorally, elicit distinct spatial activity maps ('odor maps') in the glomerular layer in the olfactory bulb. This is the first demonstration that two qualitatively distinct complex, biologically relevant odorants elicit distinct odor maps in the main olfactory bulb in mammals. Here we propose to apply modern systems neuroscience approaches to test the hypothesis that odor maps carry enough information to allow for discrimination between odortypes. In specific aims 1 and 2 we will measure odor maps in mice exposed to different behaviorally distinguishable odortypes. In specific aims 3 and 4 we will identify and functionally characterize olfactory receptors mediating odortype discrimination. Finally, in specific aim 5 we will explore the role for odortype detection of the presence of two symmetrically located odor maps in each olfactory bulb. This is to our knowledge the first comprehensive study of the neural mechanisms underlying olfactory identification of a complex natural odor by the main olfactory system in mammals. Our results will be relevant to the understanding of odor quality coding and may be relevant to aspects of human social interactions including the interaction between the mother and the infant.